Continuing the previous post, click on the picture to the right to see evidence of the changing premiums on mid and premium grade gasoline. 
The graph represents the price difference between Premium and Mid-Grade gasoline (dark blue) and the price difference for Mid-Grade and Regular gasoline from 1994-present. The green line is a 3rd-degree polynomial trend for the premium price difference and the red line is the third-degree polynomial trend for the mid-grade price differential. It seems that the price difference for both grades is increasing for about the last three years.
All data comes from the EIA Weekly Retail Gasoline and Diesel Price data.
